1. Field of Invention
The present invention relates to a semiconductor light emitting device grown on a semiconductor layer that is at least partially relaxed.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
As used herein, an “in-plane” lattice constant refers to the actual lattice constant of a layer within the device, and a “bulk” lattice constant refers to the lattice constant of relaxed, free-standing material of a given composition. The amount of strain in a layer is defined as |ain-plane−abulk|/abulk.
When a III-nitride device is conventionally grown on sapphire, the first structure grown on the substrate is generally a GaN template layer with an in-plane a-lattice constant of about 3.189 Å or less. The GaN template serves as a lattice constant template for the light emitting region in that it sets the lattice constant for all the strained device layers grown above the template layer, including the InGaN light emitting layer. Since the bulk lattice constant of InGaN is larger than the in-plane lattice constant of the conventional GaN template, the light emitting layer is compressively strained when grown over a conventional GaN template. For example, a light emitting layer configured to emit light of about 450 nm may have a composition In0.16Ga0.84N, a composition with a bulk lattice constant of 3.242 Å, as compared to the lattice constant of GaN, 3.189 Å. As the InN composition in the light emitting layer increases, as in devices designed to emit light at longer wavelengths, the compressive strain in the light emitting layer also increases.
FIG. 1 illustrates the epitaxial structure of an LED described in more detail in U.S. Pat. No. 7,547,908. A conventional low temperature nucleation layer 22 is grown directly on the surface of a sapphire substrate 20. Nucleation layer 22 is typically a low quality, non-single crystal layer such as an amorphous, polycrystalline, or cubic phase GaN layer grown to a thickness of, for example, up to 500 Å, at a temperature between 400 and 750° C. A second low temperature layer 26 is grown above nucleation layer 22. Low temperature layer 26 may be a low quality, non-single crystal layer such as an amorphous, polycrystalline, or cubic phase III-nitride layer grown to a thickness of up to 500 Å at a temperature between 400 and 750° C. Low temperature layer 26 may be InGaN, such that low temperature layer 26 increases the lattice constant of device layers 10 including an InGaN light emitting layer beyond the range of lattice constants achievable with conventional nucleation structures such as a conventional GaN template. In some examples, low temperature layer 26 is AlGaN or AlInGaN, such that low temperature layer 26 decreases the lattice constant established by nucleation layer 22 in order to decrease the tensile strain in the AlGaN light emitting region of a UV device. The light emitting active layers of such devices may be, for example, AlGaN or AlInGaN.